Ringtone: return default ringtone if actual one doesn't exist

Improve getActualRingtoneUriBySubId() and getActualDefaultRingtoneUri()
APIs to return default system ringtone when selected one doesn't exist
in media database any more.

This API enhancement is to ensure we can roll back to default ringtone
even if clip is removed, so 3rd party applications don't need to handle
such scenario.

CRs-Fixed: 671624
Change-Id: I516107e88a53eae8aea30a455f834cfd6baeea01